1. Field of the Invention
The present invention generally relates to an optical apparatus such as an optical system or an exposure apparatus, and a method of transporting or storing the optical apparatus. The exposure apparatus is preferably used in a lithography process for manufacturing micro-devices.
2. Description of the Related Art
In a conventional exposure apparatus, a wavelength of exposure light has been becoming shorter in order to obtain more precise resolution in exposure operations. Also, for the purpose of achieving a higher throughput (i.e., a processing ability per unit time) during an exposure operation, a higher power light source has been required. For example, light of g-line (436 nm), i-line (365 nm), and a KrF excimer laser (248 nm) are popular, and an ArF excimer laser (193 nm), an F.sub.2 excimer laser (158 nm) and X-rays are going to be used in the near future.
However, it is known that shorter wavelength light tends to cause a photocatalyst resulting from a reaction of contaminants included in the atmosphere with oxygen, especially when using i-line or shorter wavelength light beams. The photocatalyst generates a reaction product (clouding material) which easily attaches to the surface of the optical elements of an exposure apparatus. Namely, the optical elements, e.g., glass lenses or mirrors, will have a clouded portion that decreases the glass transparency or mirror reflectivity. For example, sulfurous acid (SO.sub.2 in water) absorbs exposure light energy in an excited state and can cause a reaction with oxygen (oxidation reaction), nitrogen and any surrounding moisture content. As a result of the reaction, ammonium sulfate (NH.sub.4).sub.2 SO.sub.4 will be produced. Ammonium sulfate, being opaque white and causing the exposure light to be scattered or absorbed, may become the clouding material on the optical elements, which degrades the optical properties of the optical elements.
Particularly, in the short wavelength region under i-line, i.e., in an excimer laser wavelength, the exposure light accelerates the chemical reaction to produce the clouding material. Also, the exposure light reacts with oxygen in the air to produce ozone. As a result, it is a known phenomenon that the remaining oxygen and the produced ozone absorb the exposure light. Hence, the reduction in the amount of the light on the wafer causes a necessity of increasing the exposure time, which means a lowering of the throughput.
It is possible to wipe or otherwise clean the deposited clouding material off of the lenses, because the material is water-soluble. However, to perform the cleaning, it is necessary to disassemble the optical elements which are held with ultra-high precision, and this will significantly increase the maintenance time and cause other difficulties.
Japanese Laid-Open Patent Application No. 9-162117 shows one solution to the problem noted above. In that document, optical systems arranged between a light source and a mask have several separated blocks, each comprising a cabinet for storing a lens barrel with optical elements. The cabinet has one window on each side to introduce and to emit the exposure light, respectively. The insides of the cabinet and the optical barrel are kept under a low-oxygen atmosphere. For this reason, the oxidation reaction of the photocatalyst is reduced so as to prevent deposition of the clouding material on the optical elements during an exposure operation.
The structure described above, however, has some points to improve. That is to say, although it is preferable to keep a higher level of airtightness in order to maintain a low-oxygen atmosphere, a differential pressure between the inside and the outside of the cabinet or the optical barrel varies (with the potential of becoming very high) according to the surrounding atmosphere.
Makers of semiconductor manufacturing apparatus supply their apparatus to manufacturing plants and research institutes located throughout the world. Therefore, they use a variety of conveying methods to transport finished products (e.g., entire exposure apparatuses) as well as units (e.g., optical system units) of the products. Air transportation is often used. Since jet planes fly at a very high altitude (e.g., at 6,000 to 12,000 m), their cabins are pressurized, but this pressure is not the same as the atmospheric pressure at the altitude on the ground. For example, if the atmospheric pressure on the ground is about 1,013 mb, the pressure at an altitude of 12,000 m is about 200 mb. In that case, the inside cabin of the flying jet plane is pressurized by about 800 mb. Further, it is known that the pressure in the cabin varies in a range of 730 to 830 mb during take off or landing. Namely, when the apparatus is transported by air, the pressure around the apparatus becomes lower than during ordinary operating times, which means the differential pressure becomes higher. That situation may cause a problem with the transported apparatus in that the windows or the optical elements may shift or, in the worst case, be damaged by the high differential pressure.
Meanwhile, in general, the inside atmosphere of clean rooms, in which the optical systems and the exposure apparatuses are put, is maintained to be under approximately the same conditions regardless of the installation, e.g., 23.+-.3.degree. C. in temperature and 45.+-.5% in relative humidity. Under these conditions, if the temperature becomes 10.degree. C. or below, the relative humidity within the airtight lens barrel or the cabinet rises to 100%, and condensation occurs. For example, when transporting the exposure apparatus by air in the wintertime, a holding cabin of an airplane can easily become under 10.degree. C. Once condensation occurs on the surfaces of the windows or the optical elements, a nucleus of water spots (i.e., extraneous matter) will remain, even if the condensation evaporates when the temperature rises. The extraneous matter may cause harmful effects on the properties of the windows or the optical elements. Particularly, for state-of-the art exposure apparatuses having an ultra-precise optical system, some improvements are needed.